Don't do assert-based tests unless in checked mode.

tests/language/async_await_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library async_await_test;
 
 import "package:unittest/unittest.dart";
 import "dart:async";
 
 main() {
@@ skipping 1708 matching lines @@
     });
 
     test("top-level getter call", () {
       f() async {
         var a = new Async();
         return await a.instanceGetter;
       }
       return expect42(f());
     });
 
+    if (!checkedMode) return;
+
     test("inside assert, true", () {                       /// 03: ok
       f() async {                                          /// 03: continued
         assert(await new Future.microtask(() => true));    /// 03: continued
         return 42;                                         /// 03: continued
       }                                                    /// 03: continued
       return expect42(f());                                /// 03: continued
     });                                                    /// 03: continued
 
     test("inside assert, false", () {                      /// 03: continued
       f() async {                                          /// 03: continued
@@ skipping 258 matching lines @@
       if (onError is BinaryFunction) {
         return onError(_error, null);
       }
       return onError(_error);
     });
   }
   Stream asStream() =>
       (new StreamController()..addError(_error)..close()).stream;
   Future timeout(Duration duration, {onTimeout}) => this;
 }